[POWERPC] Fix compile breakage for 64-bit UP configs

The rearrangements in 945feb17


("[POWERPC] irqtrace support for 64-bit powerpc") caused 64-bit
non-SMP configs to fail to compile with a message about
local_irq_save being undefined in include/linux/proportions.h.
This follows the lead of x86 in including <linux/irqflags.h> in
asm/system.h, which fixes the problem.

Signed-off-by: Paul Mackerras <paulus@samba.org>